The soon to be released MyEclipse version 10.6 will have the Eclipse Marketplace client pre-installed by default, however before it's released in several weeks here's a quick how-to guide on adding Eclipse Marketplace into MyEclipse.
Since MyEclipse 10.5 and earlier does not ship with the Eclipse Marketplace Client pre-installed the first step is to install the client from the Help > MyEclipse Configuration Center... > Software tab and clicking the '+' icon to add it to the MyEclipse configuration, as shown below.
Follow the wizard directions and allow MyEclipse to restart after the installation. Once restarted, you can add items from the Help > Eclipse Marketplace... item as you'd expect.
And that's it, now you know how to install the Eclipse Marketplace into MyEclipse. Again, this feature will be pre-installed in the coming version 10.6 of MyEclipse. If you have any questions please visit our support forums: http://genuitec.com/support